Excitation Contraction Coupling

Excitation Contraction Coupling

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Muscle fiber anatomy/structure
• Epimysium; connective tissue covering the muscle.• Muscle fibers are grouped into fascicles, covered by perimysium.• Endomysium; connective tissue covering each individual muscle fiber.• Sarcolemma; thin surface membrane of the muscle fiber.• Sarcoplasm ; semi-fluid plasm and intracellular content of the muscle fiber.• Myosin and Actin are myo-fibrils, arranged in parallel within the sarcoplasm, responsible for muscle fiber contraction.

• Myosin molecules are thick filaments.

• Thin filaments made of actin molecules bound to troponin and tropomyosin.

• A sarcomere is a sub-unit within the muscle fiber, capable of contraction. It consists of strands of actin and myosin arranged in parallel.

• Transverse tubule (T-tubule). Transverse orientation with relation to muscle fiber; structurally internal to the muscle fiber cell, but contains extracellular fluid. Muscle action potentials travel through T tubules to into the muscle.

• Longitudinal tubule ( Sarcoplamic reticulum); expand to form bulbous Terminale Cisternae at the junction with T-tubules. Longitudinal tubules do not contain sarcoplasm or extra-cellular fluid. Two Terminale Cisternaes and one interposed T-Tubule form a triad in longitudinal sections of muscle.

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Excitation-contraction coupling refers to the link between the electrical impulse nerve event and the mechanical component of muscle fiber twitch.
• Nerve impulse at the motor end plate spreads over the outer surface of the muscle fiber, through the T-tubules, coming into contact with Terminal Cisternae and then the Longitudinal tubules. • Ca++ is released from the Longitudinal tubules (Sarcoplasmic Reticulum) into the sarcoplasm.
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• Ca++ binds to troponin within the sarcoplasm, resulting in bridge formation between actin and myosin filaments.• Cross bridge formation causes actin fibers to slide relative to the thicker myosin strand.• Calcium is sequestering into the sarcoplasmic reticulum (in the presence of adenosine-triphosphate) breaking the existing bridge, relaxing the contraction.
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Ray Jurewicz
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